The following description contains references to certain compositions, articles and methods. However, these references should not necessarily be construed as an admission that such compositions, articles and methods constitute prior art under the applicable statutory provisions.
The manufacture of cemented carbides involves wet milling of powders forming a binder phase and hard constituents, drying the slurry to a powder, uniaxially pressing the powder into bodies of a desired shape, and finally, sintering. During sintering, the bodies shrink approximately 17-18% linearly. In general, the shrinkage is essentially isotropic both parallel to and perpendicular to the pressing direction. However, for submicron grades (i.e. in which essentially all of the WC grains are less than 1 xcexcm) the shrinkage is anisotropic. The shrinkage parallel to the pressing direction is larger than that perpendicular thereto.
One way to define the character of the shrinkage is by means of the K-value according to the following formula:   K  =            hs      *      wp              h      ⁢              xe2x80x83            ⁢      p      *      ws      
where
hs=the sintered height
wp=the pressed width
hp=the pressed height
ws=the sintered width
The height is defined as the dimension in the pressing direction and the width that direction perpendicular thereto.
For a completely isotropic shrinkage K=1.000. For submicron grades exhibiting anisotropic shrinkage K is less than 1.000. For the submicron grades the K value depends on the cobalt content. For submicron grades containing about 6 wt % Co, the K value is close to 1.000. For submicron grades containing 20 wt % Co, the K value is approximately 0.960.
One conventional way to characterize the average grain size of a powder is by means of the Fisher Sub-Sieve Sizer (FSSS). This apparatus employs the air permeability method in which the pressure drop over a certain amount of powder is registered and converted into an FSSS average grain size value.
U.S. Pat. No. 5,441,693 discloses the use of 0.4 xcexcm Co-powder in a submicron WC with 6.5 and 6 wt % Co, respectively.
In JP 51-126 309 the manufacture of cemented carbide with a WC grain size of 0.5-0.8 xcexcm, and 12 wt % Co with a grain size of 1 xcexcm is disclosed.
EP-A-0 380 096 discloses the manufacture of a drill shank by mixing WC and Co powder having an average particle size of 0.5 xcexcm. After sintering, the material has approximately 15-23 vol. % bond metal, or cobalt, and the average particle size of the hard phase or WC grains is 0.5-1.2 xcexcm.
As already mentioned, the shrinkage is anisotropic for submicron cemented carbide grades. In order to compensate for this non-uniform shrinkage, special pressing tools have to be made for pressing of the submicron grades, which is a large disadvantage since pressing tools are expensive to produce. Alternatively, the sintered bodies have to be subjected to an extensive grinding operation which is expensive and time consuming.
It is therefore an object of the present invention to provide a method of avoiding special pressing tools or post-sintering grinding for the manufacture of submicron cemented carbides.
According to the present invention it has now surprisingly been found that the use of a cobalt powder with essentially the same grain size as the WC-powder results in a K-value approximately equal to 1.000.
A preferred embodiment of the present invention relates to a submicron cemented carbide cutting tool in which essentially all of the WC grains are less than 1 xcexcm, preferably 0.2 to 0.9 xcexcm and with a cobalt content of 7.5 to 25 wt %, preferably 9 to 20 wt %, most preferably 10 to 15 wt %. In addition, the material contains up to 1 wt. % of conventional grain growth inhibitors such as carbides of tantalum, chromium and/or vanadium. In the case of tantalum carbide the material may contain up to 1.5 wt %.
According to preferred method of the present invention, a submicron cemented carbide cutting tool is manufactured by wet milling a slurry consisting of WC powder with an FSSS grain size, dwc, of less than 1 xcexcm, preferably 0.1 to 0.9 xcexcm, and most preferably 0.2 to 0.8 xcexcm, with Co powder in above mentioned amounts preferably with an FSSS grain size, dco, of less than 1 xcexcm such that the ratio dwc/dco0.75 less than dwc/dco less than 1.5, preferably 0.85 less than dwc/dco less than 1.3, and most preferably 0.90 less than dwc/dco less than 1.2. It is essential that the FSSS-value is determined on deagglomerated powders, since determination on agglomerated powders gives incorrect results. In addition, conventional grain growth inhibitors are added in above-mentioned amounts together with usual pressing aids.
The obtained slurry is dried to a powder with good flowability. This powder is uniaxially pressed in a pressing tool to a body of desired shape. Then, this body is sintered to form a desired article, such as a cutting tool insert. The pressing tool is the same as that used for making cemented carbides with medium to coarse WC grain size. The sintered body does not require any further grinding other than that generally necessary for corresponding medium to coarse grained grades.